“Is-ness” verses “about-ness”: Mental entities are about something else: There is a difference between “the thing” … and the thoughts about “the thing”. Thoughts are dependent upon the entity (about “the thing”). Illustration of the burglar outside of the house (the entity) vs the thoughts about the burglar. Physical entities “are” … mental entities are “about”. Philosophers call this “intentionality”

Disputable: Physical entities can be disputed … mental entities cannot be disputed. The illustration of the bugler: He may or may not exist (can be disputed) but one’s thoughts about it cannot be disputed.